add and remove trackers without supplying the whole list in libT

Description

Right now adding and removing trackers is done through tr_torrentSetAnnounceList(), which requires recreating everything. This makes the code for adding and removing trackers a bit convoluted (both in libT and the client code) and gives the calling code knowledge it doesn't need (direct access to setting tracker id, having to leave scrapes blank, etc.).

A function to add a single tracker supplying only an announce address and a tier (-1 for new tier, perhaps) would be useful. A function to remove trackers by supplying just a list of tracker id's would be equally useful.